The friction stir welding method is comprehensively described in WO 93/10935, where it emerges that a tool can consist of at least one shoulder and a probe. The probe rotates in the joint during welding, and the shoulder, which has a greater diameter than the probe, is at the same time pressed against the surface of the object for welding while cyclic movement takes place, usually rotation about the axis of symmetry. The object for welding, which in most cases consists of two workpieces which are to be welded together, is plasticized in and next to the joining line by the heat which is supplied to the object by the rotation of the probe in the joint and the rotation of the shoulder on the surface of the object for welding. As the tool is removed, the joint solidifies along the joining line, and the workpieces are joined in a strong welded joint without the material having had to be heated to melting temperature. One requirement is that the tool is made from a harder material than the object for welding. The quantity of heat supplied and the heat distribution in the object for welding are dependent on, for example, material type in the workpiece and tool and also on the geometrical design of the tool, the force applied to the shoulder and the rotation speed of the tool.
WO 93/10935 also describes a tool with double shoulders, where the two opposite sides of the object for welding are each in contact with their respective shoulder during welding, while the probe of the tool extends through the object for welding and interconnects the two shoulders. The primary shoulder can then be connected movably to the probe, while the secondary shoulder is connected firmly to the probe. It also emerges from this specification that the parts can be spring-loaded in such a way that the opposite surfaces of the object for welding are thus, via the shoulders, each subjected to a force directed at right angles in towards the surface and parallel to the rotation axis of the tool. This force is necessary in order that each shoulder will develop friction, and thus heat, when movement relative to the object for welding takes place. One disadvantage of this simple method of applying force between the shoulders and the object for welding is that the force cannot easily be controlled according to varying heat requirements—thick workpieces require more heat than thin workpieces in order to achieve a perfect welding result, and therefore greater applied force.
WO 00/02699 offers a solution to this problem by virtue of the shoulders having been provided with separate controllable force-generating devices. A control system then adjusts these forces so that they balance one another. In practice, however, it has proved to be difficult to achieve perfect welding results using this method, especially in the case of thin material for welding and when the force-generating devices have consisted of hydraulic cylinders.